The present invention relates to a power supply controlling circuit and a scanner unit constructed to have a two-system power supplied portion used to receive a power supply from an information processing apparatus such, as a computer or the like, or an external power source.
With the recent reduction in size and weight of a computer or a scanner unit, the opportunity of carrying a computer or a scanner unit with a user and using it outside or in a place where the user has gone to arises very often. For this purpose, a portable computer or a portable scanner unit has been developed by reducing size and weight of the desktop computer or the desktop scanner unit. The scanner unit is designed such that this unit is carried together with the portable computer such as a notebook computer, a mobile computer, or the like.
Among the scanner units, nowadays the scanner unit that can receive a power supply from an external computer via the interface that transfers read image data to the external computer, like the USB (Universal Serial Bus) interface, for example, has been developed.
This scanner unit aims at controlling adequately a power consumption in the scanner unit that receives a power supply via a connection cable from the computer or from the external power source to operate. Such scanner unit executes a reading operation in a normal mode when the external power source is connected, and executes the reading operation in a power saving mode when the external power source is not connected. However, the technology to control adequately the two-system power supplied portion that is used to receive the power supply from the information processing apparatus such as the computer, or the like is neither disclosed nor suggested (for example, refer to JP-A-2001-358908).
As discussed above, the technology in the related art confronts the problems given as follows.
The scanner unit in the related art receives a power supply from the computer via one USB cable when it receives the power supply via the connection cable from the computer. Therefore, normally a peak current is limited, and thus it is impossible to feed stably a sufficient power in contrast to the case where the AC power source (external power source) is used.
Recently, most of computers are equipped with at least two USB ports.
In the scanner unit, it is desired that a consumption power of the scanner unit itself should be reduced as small as possible and also a reduction in an original reading speed of and a processing speed containing the transfer of read image data to the external computer should be suppressed to the utmost by supplying the power stably to the driving portion of the scanner unit to execute a reading operation.